The present invention relates to a projection exposure apparatus for microlithography, and to a method for operating such a projection exposure apparatus.
Microlithography projection exposure apparatuses serve for producing microstructured components through a photolithographic method. In this case, a structure-bearing mask, the so-called reticle, is illuminated with the aid of a light source unit and an illumination optical unit and is imaged onto a photosensitive layer with the aid of a projection optical unit. In this case, the light source unit provides a radiation that is directed into the illumination optical unit. The illumination optical unit serves to provide a uniform illumination with a predetermined angle-dependent intensity distribution at the location of the structure-bearing mask. For this purpose, various suitable optical elements are provided within the illumination optical unit. The structure-bearing mask illuminated in this way is imaged onto a photosensitive layer with the aid of the projection optical unit. In this case, the minimum structure size which can be imaged with the aid of such a projection optical unit is determined, inter alia, by the wavelength of the radiation used. The shorter the wavelength of the radiation, the smaller the structures that can be imaged with the aid of the projection optical unit. For this reason, it is advantageous to use radiation having the wavelength of 5 nm to 15 nm, that is to say light in the wavelength spectrum of extreme ultraviolet (EUV) light, such that projection exposure apparatuses of this type are also designated as EUV projection exposure apparatuses.
In order to use radiation having the wavelength of 5 nm to 15 nm, however, it is necessary to use a luminous source plasma as light source. A light source unit of this type can be designed for example as a laser plasma source (LPP Laser Pulsed Plasma). With this type of source, a narrowly delimited source plasma is generated by a small material droplet being produced by a droplet generator and being brought to a predetermined location, where the material droplet is irradiated with a high-energy laser, such that the material undergoes transition to a plasma state and emits radiation in the wavelength range of 5 nm to 15 nm. By way of example, an infrared laser having the wavelength of 10 μm is used as the laser. Alternatively, the light source unit can also be designed as a discharge source, in which the source plasma is generated with the aid of a discharge. In both cases, alongside the desired radiation having a first wavelength in the range of 5 nm to 15 nm, which is emitted by the source plasma, radiation having a second, undesired wavelength also occurs. This involves e.g. radiation emitted by source plasma outside the desired range of 5 nm to 15 nm or, particularly when a laser plasma source is used, laser radiation which was reflected from the source plasma. Therefore, the second wavelength is typically in the infrared range having wavelengths of 0.78 μm to 1000 μm, in particular in the range of 3 μm to 50 μm. During the operation of the projection exposure apparatus with a laser plasma source, the second wavelength corresponds, in particular, to the wavelength of the laser used for generating the source plasma. When a CO2 laser is used, this is e.g. the wavelength of 10.6 μm. The radiation having the second wavelength cannot be used for imaging the structure-bearing mask, since the wavelength is too long for imaging the mask structures in the nanometers range. The radiation having the second wavelength therefore leads only to an undesired background brightness in the image plane. Furthermore, the radiation having the second wavelength leads to heating of the optical elements of the illumination optical unit and of the projection optical unit. For these two reasons, provision is made of a filter element for suppressing radiation having the second wavelength.
Spectral filters are thus used as filter elements, which spectral filters are intended to filter out undesired light components and comprise membrane films produced from a material which transmits radiation having the first desired wavelength and absorbs or reflects radiation having a second wavelength. By way of example, this can be a zirconium film having a thickness in the range of less than or equal to 500 nm, or the filter can be constructed from alternating zirconium and silicon layers. Spectral filters, so-called spectral purity filters, are known in the prior art and described in EP 1 708 031 A2, for example.
Filters with thin films have the disadvantage, however, that they can be destroyed during operation in the event of corresponding thermal loads as a result of the radiation and/or other mechanical loads, such as vibrations.
WO 2007/107783 A1 accordingly describes a method for repairing a spectral filter, wherein carbon-containing material for repairing the filter is filled into a corresponding chamber, at the end of which a spectral filter is arranged, if it is ascertained that gas is escaping from the chamber via the filter, such that it can be deduced that the filter exhibits damage. Gas sensors that can determine the corresponding gas flow are provided for this purpose.
Although such a method makes it possible to ascertain that the filter exhibits damage, and additionally opens up the possibility of the filter being repaired directly by the addition of carbon-containing material, it is necessary here to provide the corresponding prerequisites for the possibility for determining a loss of gas and the addition of a carbon-containing material into a chamber.
Furthermore, as a result of the destruction of a corresponding spectral filter there is the problem that neighboring regions of the projection exposure apparatus can be contaminated. Particularly in regions which are operated under ultrahigh vacuum as is the case in adjacent regions of the illumination system of a projection exposure apparatus, where the corresponding spectral filters are usually used, destruction of the spectral filter leads to considerable contamination of the system. Since there is additionally the difficulty that such systems are often difficult to access, the required cleaning leads to a very high outlay.
The problem area described occurs not only in the case of the spectral filters described above, but rather can generally occur in the case of corresponding filters with thin membrane films which can for example also be used for filtering debris or the like.